diff -r 083534cd7874 Lib/decimal.py
--- a/Lib/decimal.py	Thu Aug 02 17:50:06 2012 -0400
+++ b/Lib/decimal.py	Fri Aug 03 19:01:52 2012 +1000
@@ -1601,6 +1601,8 @@
 
     def __float__(self):
         """Float representation."""
+        if self.is_qnan():
+            return _math.copysign(float('nan'), (-1)**self._sign)
         return float(str(self))
 
     def __int__(self):
diff -r 083534cd7874 Lib/test/test_decimal.py
--- a/Lib/test/test_decimal.py	Thu Aug 02 17:50:06 2012 -0400
+++ b/Lib/test/test_decimal.py	Fri Aug 03 19:01:52 2012 +1000
@@ -1942,6 +1942,22 @@
         for d, n, r in test_triples:
             self.assertEqual(str(round(Decimal(d), n)), r)
 
+    def test_nan_to_float(self):
+        # Test conversions of decimal NANs to float.
+        # See http://bugs.python.org/issue15544
+        Decimal = self.decimal.Decimal
+        for s in ('nan', 'nan1234', '-nan', '-nan2468'):
+            f = float(Decimal(s))
+            self.assertTrue(math.isnan(f))
+            sign = math.copysign(1.0, f)
+            self.assertEqual(sign, -1.0 if s.startswith('-') else 1.0)
+
+    def test_snan_to_float(self):
+        Decimal = self.decimal.Decimal
+        for s in ('snan', '-snan', 'snan1357', '-snan1234'):
+            d = Decimal(s)
+            self.assertRaises(ValueError, float, d)
+
     def test_eval_round_trip(self):
         Decimal = self.decimal.Decimal